Fused-ring aromatic hydrocarbons , structures

Polycyclic Aromatic Hydrocarbon (PAH) A class of organic compounds with a fused-ring aromatic structure. PAHs result from incomplete combustion of organic carbon (including wood), municipal solid waste, and fossil fuels, as well as from... 

Fused polycyclic aromatic hydrocarbons that contain four or more rings with an angular region are carcinogenic. Their structures resemble phenanthrene. Three of die most potent carcinogens are 1,2-benzanthracene, 1,2,5,6-dibenzanthracene, and 3,4-benzpyrene. The angular region is outlined... 

Benzo[a]pyrene, a molecule with five, fused, hexagonal rings, is among the most carcinogenic of the polycyclic aromatic hydrocarbons (PAHs). Such biological activity may be related to the electronic structure of benzo[a]pyrene and its metabolites. Ionization energies of these molecules therefore have been investigated with photoelectron spectroscopy [28]. 

Some examples of different types of hydrocarbons are given in Figure 9.1. Nonaromatic compounds without ring structure are termed aliphatic, whereas those with a ring structure (e.g., cyclohexane) are termed alicyclic. Aromatic hydrocarbons often consist of several fused rings, as in the case of benzo[a]pyrene. 

Two basic issues can be distinguished here (i) the physico-chemical nature of zigzag and armchair sites at the edges of fused-benzene-ring structures, including their differences and similarities with respect to polycyclic aromatic hydrocarbon (PAH) molecules, and (ii) the reconstruction of these edges to potentially more stable structures. 

Particular attention has been focused on the toxic effects of aromatic hydrocarbons because these chemicals have proven highly carcinogenic to humans and marine life. Of greatest concern are the PAHs, which are toxic to the benthos at the ppb level. The most common compounds are shown in Figure 28.20 their structures are based on fused aromatic rings. These high-molecular-weight compoimds are very nonpolar and, hence, have low solubilities. Once in seawater, they tend to adsorb onto particles and become incorporated in the sediments. The toxicity of PAHs is enhanced by photochemical reaction with UV radiation. Photo-activated toxicity is especially problematic in shallow-water sediments, such as found in estuaries. 

Polycyclic structures (such as in polycyclic aromatic hydrocarbons), especially with more than three fused rings. 

Figure 9.11 Chemical structure of polycyclic aromatic hydrocarbons (PAHs) consisting of two or more benzene rings fused together, as shown in the PAHs naphthalene, anthracene, phenanthrene, and pyrene.

Aromatic hydrocarbons (arenes) unsaturated compounds hexagonal ring structure single and multiple fused rings... 

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